Industrial fermentation methods predominantly use glucose as a feedstock for the production of a multitude of end-products, including enzymes, proteins, amino acids, organic acids, sugar alcohols, pharmaceuticals and other biochemicals. In many applications glucose is produced from the enzymatic conversion of substrates comprising starch and cellulose (e.g., whole milled cereal grains). The processing of starch to produce glucose generally involves two steps, namely liquefaction of granular starch and saccharification of the liquefied starch to produce glucose. Further steps may include purification and isomerization, e.g., when the desired end-product is a purified dextrose or fructose, or fermentation and distillation, e.g., when the desired end-product is an alcohol (e.g., ethanol).
Liquefaction converts a slurry of starch polymer granules into a solution of shorter chain length dextrins of lower viscosity. The saccharification step further converts those shorter-chain dextrins into glucose. Commonly, the starch is liquefied by exposure to an elevated temperature and enzymatic bioconversion. A common enzymatic liquefaction process involves adding a thermostable bacterial alpha (α)-amylase (e.g., SPEZYME® FRED or SPEZYME® XTRA (Danisco US, Inc, Genencor Division) or TERMAMYL® SC or TERMAMYL™120L (Novozymes)) to a slurry comprising a substrate that includes granular starch. The pH is adjusted to between 5.5 to 6.5 and the temperature is elevated to greater than 90° C. The starch is first gelatinized and then exposed to the saccharifying enzymes. Typically, saccharification takes place in the presence of glucoamylase enzymes such as glucoamylase from Aspergillus niger (e.g., OPTIDEX® L-400 (Danisco US, Inc. Genencor Division)) at a more acidic pH than that used in the liquefaction step. The pH of a typical saccharification step is around pH 4.0 to 5.0. The resulting sugars are then fermented to provide the desired end-products (i.e., ethanol). In the process of producing ethanol, side-products and waste-products such as distillers dried grains and solubles (DDGS) are produced and used for feed. Further, the resulting liquid from the process (i.e., the thin stillage) is recycled by mixing it with slurry.
A number of variations exist for the liquefaction and saccharification of a starch substrate. However, a need continues to exist for advances in starch liquefaction, saccharification, and fermentation.